The if-then Statement
The if-then statement is the most basic of all the control
flow statements. It tells your program to execute a certain section of code only if a particular test evaluates to true. For example, the Bicycle class could allow the
brakes to decrease the bicycle's speed only if the bicycle is already
in motion. One possible implementation of theapplyBrakes method could be as follows:
void applyBrakes() {
// the "if"
clause: bicycle must be moving
if (isMoving){
// the
"then" clause: decrease current speed
currentSpeed--;
}
}
If this test evaluates
to false (meaning that the bicycle is not in motion),
control jumps to the end of the if-then statement.
In addition, the opening
and closing braces are optional, provided that the "then" clause
contains only one statement:
void applyBrakes() {
// same as above, but
without braces
if (isMoving)
currentSpeed--;
}
Deciding when to omit
the braces is a matter of personal taste. Omitting them can make the code more
brittle. If a second statement is later added to the "then" clause, a
common mistake would be forgetting to add the newly required braces. The
compiler cannot
catch this sort of error; you'll just get the wrong results.
The if-then-else Statement
The if-then-else statement provides a secondary path of execution
when an "if" clause evaluates to false. You could use an if-then-else statement in theapplyBrakes method to take some action if the brakes are
applied when the bicycle is not in motion. In this case, the action is to
simply print an error message stating that the bicycle has already stopped.
void applyBrakes() {
if (isMoving) {
currentSpeed--;
} else {
System.err.println("The bicycle has already stopped!");
}
}
The following program, IfElseDemo, assigns a grade
based on the value of a test score: an A for a score of 90% or above, a B for a
score of 80% or above, and so on.
class IfElseDemo {
public static void
main(String[] args) {
int testscore = 76;
char grade;
if (testscore >=
90) {
grade = 'A';
} else if (testscore
>= 80) {
grade = 'B';
} else if (testscore
>= 70) {
grade = 'C';
} else if (testscore
>= 60) {
grade = 'D';
} else {
grade = 'F';
}
System.out.println("Grade = " + grade);
}
}
The output from the
program is:
Grade = C
You may have noticed
that the value of testscore can satisfy more than
one expression in the compound statement: 76 >= 70 and 76 >= 60. However, once a
condition is satisfied, the appropriate statements are executed (grade = 'C';) and the remaining conditions are not evaluated.
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